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Effect of bladder volume changes and verification of CTV on CBCT for rectal cancer patients

Andersson, Lina (2016) MSFT01 20161
Medical Radiation Physics, Lund
Medical Physics Programme
Abstract
Background and purpose: During radiotherapy, rectal cancer patients show inter-fractional internal motion that effects the delivered dose distribution. The purpose of this work is to study I) the inter-fractional bladder volume change, II) the effect of bladder volume change on bowel dose distribution, III) the effect of bladder optimization on the relationship between bladder volume and bowel dose and IV) the possibilities and difficulties of validating the clinical target volume (CTV) using a surrogate on cone-beam computed tomography (CBCT) scans. An additional purpose is to find the most appropriate surrogate of the CTV to use in the validation.

Material and methods: Twenty-eight rectal cancer patients treated with preoperative... (More)
Background and purpose: During radiotherapy, rectal cancer patients show inter-fractional internal motion that effects the delivered dose distribution. The purpose of this work is to study I) the inter-fractional bladder volume change, II) the effect of bladder volume change on bowel dose distribution, III) the effect of bladder optimization on the relationship between bladder volume and bowel dose and IV) the possibilities and difficulties of validating the clinical target volume (CTV) using a surrogate on cone-beam computed tomography (CBCT) scans. An additional purpose is to find the most appropriate surrogate of the CTV to use in the validation.

Material and methods: Twenty-eight rectal cancer patients treated with preoperative radiotherapy in the period February 2015 to January 2016 were included in the study. All treatments were delivered with RapidArc, using two arcs. Each patient had a planning CT scan and weekly CBCT were acquired the first three, four, five and six weeks for 23, 1, 1 and 3 patients, respectively. The bladder and the bowel was delineated on the CBCT scans and transferred to the CT scan Treatment plans not including a bladder optimization were re-optimized, to investigate possibilities with lowering the bowel dose. In accordance with the latest local guidelines, the re-optimized treatment plans included a bladder optimization and delivered the dose with three arcs instead of two.

A surrogate of the CTV was delineated on the CT and compared with corresponding structure on the CBCT. The surrogate was defined as the rectum where rectum and mesorectum could be distinguished and as the mesorectum elsewhere. Caudally from the rectum, the surrogate was defined by the circumference of the levator ani. No surrogate was delineated cranial from the rectum. The surrogate was divided into an upper, mid and lower section during the validation.

Results: The median bladder size was significantly smaller on the CBCT than on the CT. Out of the 28 patients, 13 patients had bladders consistently smaller and larger on the CBCTs. Six patients had consistently larger bladder volumes on the CBCTs than on the CT. For 9 patients, the relationship between the bladder volume on the CT and CBCTs varied. The bladder volumes on the CBCTs ranged from 0.1 to 3.5 times the bladder volume on the corresponding CT, with the majority (81/96) in the range between 0.3 and 2.0. The change in bladder filling did not result in a violation of the bowel constraint V45Gy<195 cm3 for any patient. The re-optimized treatment plans resulted in lower bowel doses, without compromising PTV coverage. The bladder optimization was not proven (p=0.46) having an effect the relationship between V45Gy and bladder volume change.

The image quality was sufficient for a validation of the surrogate on 94/95 CBCT scans. The CBCT scan where the surrogate could not be validated suffered from major artifacts due to internal gas. The delineated surrogate did not extend as far cranially as the gross tumor volume (GTV) or the primary CTV for 12/28 and 19/26 patients, respectively. The variation of surrogate was within 5 mm in the mid and upper section on 40/70 and 56/90 CBCTs, respectively. Cranial-caudal shifts in the position of the sigmoideum and internal gas challenged the validation of the surrogate.

Conclusion: There is a correlation between bladder volume and bowel radiation dose. However, the dose constraint for the small bowel was not violated for any of the patients in this study. The relation between bowel dose and bladder volume is highly individual and not proven being effected by bladder optimization. The bowel dose can be reduced by optimizing the plans according to the latest local guidelines. The CTV can be validated on CBCT using a surrogate. The surrogate can be defined somewhat general caudal from the cranial border of rectum but not cranially from rectum. The variation of the surrogate is usually within 5 mm. (Less)
Popular Abstract (Swedish)
Ändtarmscancer är en av de vanligast förekommande cancersjukdomarna. Tillsammans med tjocktarmscancer är det den tredje vanligaste cancersjukdomen i världen. Den behandlas vanligen med kirurgi, under vilken en stor del av ändtarmen och dess omgivande fettvävnad tas bort. För att minska risken för återfall kan operationen kompletteras med strålbehandling, där patienterna oftast strålbehandlas innan de opereras. Under strålbehandlingen riktas då till området kring tumören, d.v.s. till ändtarmen och omkringliggande mjukvävnad. Den totala mängden strålning delas upp i flera fraktioner, så att patienten får strålbehandling dagligen 5 dagar i veckan i 1 eller 5 veckor.

Utöver att minska risken för återfall, medför den kompletterande... (More)
Ändtarmscancer är en av de vanligast förekommande cancersjukdomarna. Tillsammans med tjocktarmscancer är det den tredje vanligaste cancersjukdomen i världen. Den behandlas vanligen med kirurgi, under vilken en stor del av ändtarmen och dess omgivande fettvävnad tas bort. För att minska risken för återfall kan operationen kompletteras med strålbehandling, där patienterna oftast strålbehandlas innan de opereras. Under strålbehandlingen riktas då till området kring tumören, d.v.s. till ändtarmen och omkringliggande mjukvävnad. Den totala mängden strålning delas upp i flera fraktioner, så att patienten får strålbehandling dagligen 5 dagar i veckan i 1 eller 5 veckor.

Utöver att minska risken för återfall, medför den kompletterande strålbehandlingen en rad biverkningar till följd av att strålkänsliga, friska organ exponeras för strålning. Tunntarmen är ett av dessa organ. Då tunntarmen ligger nära ändtarmen är tunntarmen nästintill oundviklig att inte bestråla. För att minska biverkningarna används olika metoder som minskar den bestrålade tunntarmsvolymen, varav en är att låta patienten ha en full urinblåsa. Av naturliga skäl kommer urinblåsans volym dock att variera under behandlingens förlopp, vilket innebär att den bestrålade tunntarmsvolymen även kommer att variera. I detta arbete undersöktes huruvida det går att på ett enkelt och snabbt sätt översätta en förändrad urinblåsvolym till förändrad tunntarmsstråldos. Det undersöktes även om det finns det ett mönster i ändringen av urinblåsans volym, d.v.s. om den ökar eller minskar under strålbehandlingen gång.

Resultaten visade att urinblåsans volym vanligtvis är större i början av strålbehandlingsprocessen än under efterföljande strålbehandlingsfraktioner. Minskningen i urinblåsvolym resulterade dock inte i att tunntarmen utsattes för mer strålning än vad som rekommenderas, för någon av patienterna i studien. Ytterligare visade resultaten en klar koppling mellan urinblåsans volym och tunntarmsdos. Däremot finns det inte en generell formel för att översätta en minskning i blåsvolym till en ökning i tunntarmsdos då översättning är högst individuell för varje patient.

För att strålningen ska nå dit man vill, d.v.s. till ändtarmen och omkringliggande mjukvävnad, kontrolleras patientens position genom att ta röntgenbilder innan strålbehandlingen. På strålbehandlingsavdelningen i Herlev (Danmark) tas tvådimensionella röntgenbilden innan varje strålbehandlingsfraktion och patientens position kontrolleras genom att kolla på benstrukturerna i bilderna. Om det är möjligt vill man i framtiden byta ut de tvådimensionella röntgenbilderna mot tredimensionella bilder och verifiera patientens position genom att, utöver benstrukturer, även kolla på ändtarmen och annan mjukvävnad i bilderna. I detta arbete undersöktes om bildkvaliteten på de tredimensionella röntgenbilderna är tillräckligt bra för att man ska kunna urskilja ändtarmen och annan mjukvävnad. Ytterligare undersöktes hur mycket ändtarmen och omkringliggande mjukvävnad rör sig mellan strålbehandlingsfraktionerna.

Resultaten visade att bildkvaliteten var tillräckligt bra för att urskilja ändtarmen på 94 av 95 bilder. Något som försvårar möjligheten att urskilja mjukvävnaden är när patienten hade mycket luft i ändtarmen, då luften skapar mörka stråk i bilderna. Ändtarmen rör sig vanligtvis inom 5 mm från positionen den har när strålbehandlingen planläggs. (Less)
Please use this url to cite or link to this publication:
author
Andersson, Lina
supervisor
organization
course
MSFT01 20161
year
type
H2 - Master's Degree (Two Years)
subject
language
English
id
8890962
date added to LUP
2016-09-07 11:55:11
date last changed
2017-01-09 16:28:25
@misc{8890962,
  abstract     = {Background and purpose: During radiotherapy, rectal cancer patients show inter-fractional internal motion that effects the delivered dose distribution. The purpose of this work is to study I) the inter-fractional bladder volume change, II) the effect of bladder volume change on bowel dose distribution, III) the effect of bladder optimization on the relationship between bladder volume and bowel dose and IV) the possibilities and difficulties of validating the clinical target volume (CTV) using a surrogate on cone-beam computed tomography (CBCT) scans. An additional purpose is to find the most appropriate surrogate of the CTV to use in the validation.

Material and methods: Twenty-eight rectal cancer patients treated with preoperative radiotherapy in the period February 2015 to January 2016 were included in the study. All treatments were delivered with RapidArc, using two arcs. Each patient had a planning CT scan and weekly CBCT were acquired the first three, four, five and six weeks for 23, 1, 1 and 3 patients, respectively. The bladder and the bowel was delineated on the CBCT scans and transferred to the CT scan Treatment plans not including a bladder optimization were re-optimized, to investigate possibilities with lowering the bowel dose. In accordance with the latest local guidelines, the re-optimized treatment plans included a bladder optimization and delivered the dose with three arcs instead of two.

A surrogate of the CTV was delineated on the CT and compared with corresponding structure on the CBCT. The surrogate was defined as the rectum where rectum and mesorectum could be distinguished and as the mesorectum elsewhere. Caudally from the rectum, the surrogate was defined by the circumference of the levator ani. No surrogate was delineated cranial from the rectum. The surrogate was divided into an upper, mid and lower section during the validation.

Results: The median bladder size was significantly smaller on the CBCT than on the CT. Out of the 28 patients, 13 patients had bladders consistently smaller and larger on the CBCTs. Six patients had consistently larger bladder volumes on the CBCTs than on the CT. For 9 patients, the relationship between the bladder volume on the CT and CBCTs varied. The bladder volumes on the CBCTs ranged from 0.1 to 3.5 times the bladder volume on the corresponding CT, with the majority (81/96) in the range between 0.3 and 2.0. The change in bladder filling did not result in a violation of the bowel constraint V45Gy<195 cm3 for any patient. The re-optimized treatment plans resulted in lower bowel doses, without compromising PTV coverage. The bladder optimization was not proven (p=0.46) having an effect the relationship between V45Gy and bladder volume change.

The image quality was sufficient for a validation of the surrogate on 94/95 CBCT scans. The CBCT scan where the surrogate could not be validated suffered from major artifacts due to internal gas. The delineated surrogate did not extend as far cranially as the gross tumor volume (GTV) or the primary CTV for 12/28 and 19/26 patients, respectively. The variation of surrogate was within 5 mm in the mid and upper section on 40/70 and 56/90 CBCTs, respectively. Cranial-caudal shifts in the position of the sigmoideum and internal gas challenged the validation of the surrogate.

Conclusion: There is a correlation between bladder volume and bowel radiation dose. However, the dose constraint for the small bowel was not violated for any of the patients in this study. The relation between bowel dose and bladder volume is highly individual and not proven being effected by bladder optimization. The bowel dose can be reduced by optimizing the plans according to the latest local guidelines. The CTV can be validated on CBCT using a surrogate. The surrogate can be defined somewhat general caudal from the cranial border of rectum but not cranially from rectum. The variation of the surrogate is usually within 5 mm.},
  author       = {Andersson, Lina},
  language     = {eng},
  note         = {Student Paper},
  title        = {Effect of bladder volume changes and verification of CTV on CBCT for rectal cancer patients},
  year         = {2016},
}